1.Field of the Invention
The invention is based on an electrical machine or a method for operating an electrical machine.
2.Description of the Prior Art
From German Patent Disclosure DE 10 2004 027 635 A1, one such electrical machine is known as a drive for a hand-held or stationary electric tool, and its drive unit has an electronically commutatable motor with a permanent-magnet-excited rotor, and its stator is operated by means of a motor controller in such a way that the motor, in a first rpm range, operates in a voltage-controlled mode and in a second rpm range, which adjoins the first rpm range in the direction of a higher rpm, is triggered in accordance with a field attenuation mode. The field attenuation is attained by means of a phase displacement between the magnetomotive force of the rotor and of the stator, and advance commutation of the stator current is effected. This mode of operation, in which in the field attenuation rpm range the exciter current of the stator leads ahead of the pole wheel voltage, can be achieved inexpensively by comparatively simple means and can be employed when stringent demands are not made of the guidance of the electrical machine, and particularly when highly dynamic setting of the transverse axis flow forming the torque can be dispensed with.
In principle, the idling rpm of electronically commutated (EC) motors is defined by the design of the winding and the magnitude of the voltage of the supplying direct current source. When voltage is supplied constantly, the maximum rpm, or the idling rpm, of the motor is thus defined. To increase the motor rpm still further, a field attenuation is necessary, in which the amount of the magnetic flux linked with the stator winding is reduced. To that end, it is known to attain the field attenuation by means of a stator current component which generates a magnetomotive force in the stator winding parallel to the magnetomotive force of the exciter. In the literature, this component is called the longitudinal or d-axis current, referred to the main axis of the rotor. This d-axis component is as a rule oriented such that it is counter to the magnetomotive force of the exciter, and thus the resultant magnetomotive force of the exciter in the main exciter axis is reduced.
The resultant d-axis magnetomotive force induces a voltage in the armature winding of the machine that leads ahead of the magnetomotive force by 90°. This induced voltage is in phase with a current that generates an armature magnetomotive force in the transverse (q) axis of the rotor and with it converts electrical power into mechanical power. On this basis, a field attenuation or field-oriented regulation is possible in which the two stator current components can be adjusted independently of one another. With the q-axis current, the torque is adjusted, and with the d-axis current, the induced voltage in the machine can be reduced so far that rotary speeds far above the natural idling rpm of the machine are attainable. One such regulation is described for instance in German Patent DE 197 25 136 C2.